Added footnote support and improve balancing logic for parallel typesetting.

[no-vici]

Introduced some features for footnote handling, improved frame alignment logic, and support for customisable glue and spacing.

[alerque]

Wow thanks for jumping in and contributing.

I don't have time to give this a proper review this moment, but I certainly will. Just some preliminary feedback (and these are all things we can help fix and address, just FYI):

1. You seem to have your editor set to override existing indentation with tabs. We're using 3-space indent. You might want to enable an EditorConfig aware extension in your editor so it automatically adjusts to the right coding style for projects.

2. The Lua styling (including the above spacing fixes) are easily fixed after the fact using stylua. It will deterministically reformat all the Lua code in the project, so don't bother fixing it manually, we'll just run that and get this fixed up.

3. Commit messages need to follow Conventional Commits guidelines. You can look at the history for ideas of the scope names we use. Again these can be fixed up after the fact, just a note for future reference.

4. We tend to prefer granular commits. Do small increments and label them as features, fixes, refactors, docs, etc. One commit per task scope. We can work on splitting this up, and I can help do that while reviewing it, but as you work just keep that in mind, it is usually easier to keep more granular commits than you may end up wanting and squash them than it is to split up later if you do too big, so err or the side of many commits when in doubt.

[alerque]

I just tried to fix some of the issues noted, but ran into one more thing: when you open PRs, it helps a lot if you start a new branch on your fork before opening the PR. That way you can allow us (maintainers) to contribute to the PR. As it is this is opened against your own forks master branch, which you cannot let us push to.

I just pushed my modifications of your commits to this branch. You can reset your branch to the latest commit there, or run stylua and split up commits yourself, or close this PR and open one on a branch so I can just push directly to it, or whatever....